Field of the Invention
The invention relates to a circuit for generating a further voltage from a supply voltage.
The further voltage may be greater or less than the supply voltage. Such voltage generator circuits are often used in integrated circuits in order to supply certain parts of the circuit with an operating or control voltage that differs from the supply voltage to the integrated circuit. For example, in dynamic semiconductor memories (DRAMs) with a memory cell that includes a selection transistor and a storage capacitor, the selection transistor may be an n-channel MOS transistor. In order to enable reliable writing of the information to be stored, the trigger voltage of a word line, by which the gate terminal of the selection transistor is controlled, must be above the supply voltage. For the sake of reliability, that increased voltage must moreover not exceed a maximum value. Circuits for voltage generation are also used if, for instance for reasons of standardization, circuits are supplied with a fixed supply voltage (such as 5 V), yet the internal circuits are operated at a reduced operating voltage (such as 3.3 V).
In order to generate the increased word line voltage in DRAMs, booster circuits are known which operate by the principle of charge displacement. The high voltage is generated by slide capacitors that are precharged in the resting phase of the DRAMs. However, a disadvantage of such boosters is that the slide capacitors are charged and discharged completely upon each operating cycle. Since the increased voltage must be reliably generated in the fluctuating range of the supply voltage, the slide capacitors must be constructed for the lower limit of the fluctuation range. They are accordingly relatively large and thus consume a great deal of power. Limiting the maximum word line voltage for a supply voltage at the upper fluctuation range, moreover requires complicated circuits.
In an article entitled "An Experimental 1.5-V 64-Mb DRAM" by Y. Nakagome, in the IEEE Journal of Solid-State Circuits, Vol. 26, No. 4, April 1991, a charge pump is used to generate the increased voltage. The word lines are charged from the storage capacitor by charge transfer in the reading or writing phase and are brought to the increased level with a smaller, two-stage charge slider. No provisions with which the maximal voltage could be limited are shown.